Recovering addicts have relapsed after taking a few tramadol for pain, so it's definitely possible to relapse after getting a little taste of the high via Vicodin. Also, I don't think that hydrocodone would do much for a migraine headache. There are certain meds for that, and they aren't opioids. I have heard of some hospitals using IV hydromorphone in some extreme situations, but people suffering from migrains usually take non-opioid meds to treat these headaches.
What kind of back pain does he have? Again, hydrocodone might not be the proper med for his condition. Muscle relaxers may work better if his pain is coming from muscle cramps or spasms, or something like gabapentin if it's nerve pain.
It seems like your friend is trying to justify using the vicodin, so he might already be on the road to relapse. They say that you relapse long before you actually take the substance, and this may be one of those cases. Tell your friend to look into proper treatment for his migraines and back pain, and to tell the doctor about his history of addiction so that they don't toss some opiates at him, which would further justify him using them, since it would be 'doctors orders.'